Looks like they only include a tiny preview image in the (raw) file. If that's the case there's not much PM can do about this. An option for you could be to convert the images to DNG and have the DNG converter embed a full resolution preview. Then PM will be able to show it to you.

(If you're on a Mac, another option is to enable Apple's raw rendering within PM (Preferences|RAW), but this will work only if Apple already supports your camera. Also, it will be much slower (for everything!) than without the raw rendering)